The invention relates to a desaturation circuit for an IGBT and to a method for desaturating an IGBT.
Insulated-gate bipolar transistors (IGBTs) combine the principle of a bipolar transistor with a wattless driving of a MOS (metal oxide semiconductor) field effect transistor. An electron current is conducted via a lateral or vertical MOS channel for driving the IGBT. This electron current leads to the injection of holes from a pn junction formed at a rear of the IGBT and polarized in the forward direction. As a result, the low conductivity of a voltage-accepting layer can be increased by several orders of magnitude in the turned-on state of the IGBT by injecting an electron hole plasma. When the IGBT is turned off, however, this conductive plasma must be removed from the active zone which is bound to lead to turn-off losses since, during a voltage rise between emitter and collector of the IGBT, a depletion current or part-current for removing the electron hole plasma continues to flow.
In known NPT (non-punch through) IGBTs with planar cell geometry such as IGBT2 by Infineon Technologies, the variation of gate voltage and collector voltage when the IGBT is turned off is determined by the impedance of the gate circuit, by a gate resistance within the driver, series resistances in modules and on the chip and by parasitic capacitances of the IGBT. A reduction in this impedance leads to a quicker discharge of the gate to a Miller plateau; the Miller plateau becomes shorter and the voltage at the collector rises more quickly. This behavior can be influenced within wide ranges by changing the impedance or a magnitude of the gate current in order to produce a corresponding reaction of the IGBT.
IGBTs of the latest IGBT generations with trench cells and field stop, such as e.g., IGBT3 by Infineon Technologies, however, illustrate a behavior which differs from the above behavior. Such IGBTs are characterized by very low on-state values with increased current densities which is achieved by improvements in the charge carrier distributions of electrons and holes. Lower on-state values are achieved by increased flooding of the component with electrons and holes in the conducting state. At the same time, however, the drive characteristics and the controllability of current and voltage variations change particularly when turning off under inductive loads such as in motor drives. The switching behavior is no longer determined by the parasitic capacitances and driver impedances alone. Instead, a voltage rise at the collector when turning off the IGBT can no longer follow the discharge of the gate capacitance when the impedances become smaller which is why the gate is discharged below the Miller plateau, i.e. the gate voltage when load current is flowing. Since the load current continues to flow during this time, it is fed by charge carriers stored in the IGBT. This also determines an increase in the collector voltage by the removal of the charge carriers stored in the IGBT and cannot be accelerated by greater discharge of the gate with lower gate impedance. However, such behavior impedes a delayless feedback of a temporal change in the collector current or overvoltages at the collector since the gate must firstly be charged up to the Miller plateau again until this novel IGBT reacts. In addition, a voltage rise in the IGBT cannot be accelerated which was possible in IGBTs of known prior generations by reducing the gate impedance. However, such acceleration in the voltage rise is desirable for fast switching applications.
To solve the above problems, it is known to increase impedances in the gate circuit of an IGBT to such an extent that it turns off with a slow current drop. Lowering the gate voltage below the Miller plateau was compensated for in this case by means of special gate control circuits in order to provide for a delayless feedback. Current units were limited to low di/dt by suitable adjustments of the IGBT.
EP 0 898 811 B1 describes a method for changing the turn-off behavior of a known IGBT of a generation such as IGBT2. For this purpose, the Miller capacitance is reduced by increasing the collector voltage before the turning off in order to be able to control a subsequent change in time of the collector voltage dV/dt. This is achieved by first reaching the Miller plateau because the delay time to the steeper voltage rise is thus shortened. The collector voltages are set to high voltages such as, for example, 200 V for reducing the Miller capacitance.
DE 102 06 392 A1 proposes a stepped turn-off of the IGBT.
For these and other reasons, there is a need for the present invention.